The neurobehavioral effects of 2,4,5,2',4',5'-hexabromobiphenyl (HBB) and Firemaster FF-1, a commercial mixture of polybrominated biphenyls (PBB), of which HBB is the major component, have been shown to be different when given in amounts equating HBB content. The present study repeats and extends observations of neurobehavioral effects of equal doses of HBB or FF-1. Rats were trained to press a lever in standard operant conditioning chambers to avoid an unsignaled presentation of electric footshock and to escape shock when it is presented (unsignaled continuous avoidance) until a stable baseline of responding is achieved. Dosing (10 mg/kg of FF-1, 10 mg/kg of HBB, or corn oil vehicle) was started. Effects on performance of the avoidance schedule were evaluated.